Two-year-old JJ Lawrence was killed by a second person, probably his mother, the defence in the child murder trial has said.

The accused, Joel Loffley, finally revealed his hand yesterday with defence lawyer Roger Chambers saying his client had been the victim of a "selective prosecution" and "imagination" on the part of the Crown.

Mr Chambers focused on the evidence of Jessica Peat, the partner of Loffley's brother Gabriel, who was in the house the morning JJ was killed.

The Crown says Loffley assaulted JJ then showered him, dressed him, put him to bed and left the house.

Ms Peat had testified that, while Loffley was out, she heard JJ's door being opened and then closed a few minutes later.

"That is when the defence says the assault on JJ took place," Mr Chambers said.

Ms Peat said she saw Loffley "freak out" when he returned and discovered JJ not breathing and wearing only a nappy.

Mr Chambers said the medical evidence was that JJ's horrendous injuries would have caused the boy's death "pretty quickly", but his body was still warm when ambulance officers arrived.

"The assault on JJ was not earlier on before Joel and Gabriel left the house. It is my submission that someone went into JJ's room, damaged JJ, and removed clothing."

Mr Chambers said it was not known who this person was but he noted JJ's mother Josephine Lawrence had made an admission to Loffley.

He also pointed to Ms Peat's evidence that there was "giggling" coming from Lawrence and her family as they were waiting outside the house, immediately after the death.

"People who are angry can sometimes have the strength of demons. Did Josie have a meltdown for whatever reason?" Mr Chambers asked the jury.

Earlier, anticipating the defence case, prosecutor Phil Hamlin asked: "Why on Earth would this mother want to kill her child?"

Mr Hamlin asked who was violent and controlling in the relationship and who had motive to kill JJ.

He pointed to lies Loffley told that distanced himself from any harm to JJ, including telling police the boy's broken arm happened when he was staying with family.

Josephine Lawrence had testified that Loffley had admitted he might have caused it.

The Crown alleges that on November 14, 2011, Loffley struck JJ at their Auckland home while Ms Lawrence was out of the house.

Doctors said his pancreas and liver had been pushed back on to his spine with such force they split in half. JJ was also found to have cannabis in his system, which Ms Lawrence testified Loffley used to give to the boy.

Loffley found JJ in his bed not breathing.

Justice Patrick Keane will sum up today before asking the jury to consider its verdict.